Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line's resolution is a result of the directivity of the associated transmit and receive beam pair.
In B-mode ultrasound imaging, two-dimensional images of tissue are created in which the brightness of a pixel is based on the intensity of the echo return. The outputs of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
In B-mode ultrasound imaging, the signal-to-noise ratio (SNR) is a key measure of image quality. A higher SNR often implies increased penetration at a given imaging frequency. Alternatively, SNR can be traded off for improved resolution by imaging at higher frequency. Conventional ultrasound scanners often support two or more user-selectable transmit frequencies for B-mode imaging. Higher frequencies can be used for the "easy" patient type for maximum resolution, whereas lower frequencies are used to achieve better penetration for the "difficult" patient type. The different transmit frequency waveforms may also differ in the number of pulses or burst length. Generally longer burst lengths (increased dosage) are used to obtain better penetration at the expense of resolution. The preset or default transmit frequency for a given probe is usually chosen based on application type. In practice, however, the patient-to-patient variations are so great that the preset transmit frequency will not always be optimal.
While state-of-the-art scanners provide the user with selectable imaging parameters such as transmit frequency, acoustic output, TGC curve, dynamic range and frame averaging level, all of which can significantly affect B-mode sensitivity and penetration, the sonographer usually does not have the time (or training) to fully optimize all these controls. If the patient appears difficult to scan, the sonographer may resort quickly to the lowest transmit frequency, or switch to a lower frequency probe or to a different machine. On the other hand, if the current scan is acceptable, the user may not bother to select a higher transmit frequency for better resolution. To improve the ease-of-use and efficiency of the scanner, there is a need to automate selection of some of the basic imaging parameters based on the actual image data.